1. Field of the Invention
This invention relates generally to electron beam lithography systems, and more specifically to an improvement to cassettes for holding reticles to be written upon by an electron beam system.
2. Prior Art
Electron beam ("E-beam") lithography systems such as the Perkin-Elmer "MEBES" system write an electron beam on blank glass reticles to develop patterns for masks to be used in fabricating integrated circuits. A blank reticle is coated with electron-beam sensitive resist over a layer of chrome and inserted into a cassette, which, together with other cassettes, is loaded into a magazine. From the magazine, cassettes are loaded one at a time onto an X-Y stage under the electron beam in a high vacuum work chamber inside the E-beam system. Each cassette has a V-shaped notch formed in the back face and another in the right side for aligning the cassette against precision alignment pins fixed to the stage. The V-shaped notches have a hard coating to reduce wear which would otherwise affect the dimensional integrity of the cassette.
After a cassette loaded with a reticle is positioned on the stage, an electron beam is scanned rapidly across the blank reticle to expose resist according to a prespecified pattern to write a mask on the reticle. The cassette is then withdrawn from the stage and the reticle is removed from the cassette. The resist is subsequently developed to expose underlying chrome areas to be etched away. Afterwards, there remains a mask negative image which will be transferred repeatedly onto semiconductor substrates to manufacture integrated circuit devices.
Since present day electron beam lighography systems write mask pattern features as small as 0.14 microns, it is apparent that any dust or extraneous particulate matter appearing on the surface of the reticle will adversely affect the resulting mask product. In other words, any particulate matter which deposits on the reticle blank before or during writing is likely to distort features in the pattern being written on the reticle. The smaller the feature size, the smaller the size of particles that it is critical to eliminate from the work chamber and reticle blank.
Although cassettes and recticles are handled and processed under cleanroom conditions, it has been found that (notwithstanding ongoing enhancements), the MEBES system's performance is degraded when reticles are contaminated with particles generated by, and inside of, the system. Since the blank reticle-holding cassettes are made of untreated aluminum, during normal operation of the machine, frictionally contacted cassette surfaces are gradually worn down and the resulting small particles of alumninum are picked up by the guide rollers inside the machine. Referring to FIG. 1, as the roller surfaces become saturated as shown in part 1 of the curve increasing number of particles begin to precipitate onto the reticles, and this correspondingly inreases the rate of mask rejection for particle contamination. Present day E-beam lithography machines rent for around $3,000.00 per hour, and take up to four hours to write some masks. After this amount of writing time, a defect causing a mask to be rejected effectively costs the E-beam system operator around $12,000.00 in lost production time. To reduce the level of particle contamination, approximately every six months (at the end of part 1 of the curve in FIG. 1) E-beam machines must be disassembled and the work chamber opened to clean the cassette loading guide and transport rollers located throughout the loading and work chambers. Barring unforeseen complications, cleaning requires stopping production for a total of approximately 5 days. Furthermore, the E-Beam work chamber, which is normally evacuated to a vacuum of 2.times.10e-07 to 10.times.10e-08 Torr, contains fragile components susceptible to magnetic fields, which become exposed to the air yet must be kept extremely clean to permit restoration of the high vacuum environment, which requires up to three days. Recalibration requires another two to three days. Although this cleaning procedure reduces the number of particles and corresponding mask rejects to their original levels, and restarts the cycle of particle accumulation, it has been found that even after the machine has been cleaned, some particles remain on the rollers and precipitate out of the rollers at a declining rate as indicated by part 2 of the curve in FIG. 1. At the end of part 2 of the curve, a new accumulation cycle begins and continues through part 3 of the curve. The typical production loss for a given E-beam machine averages 65,000 dollars per year.